Chemical vapor deposition of fluorocarbon polymer thin films

ABSTRACT

Provided are methods for forming a fluorocarbon polymer thin film on the surface of a structure. In one method, a monomer gas is exposed to a source of heat having a temperature sufficient to pyrolyze the monomer gas and produce a source of reactive CF 2  species in the vicinity of the structure surface. The structure surface is maintained substantially at a temperature lower than that of the heat source to induce deposition and polymerization of the CF 2  species on the structure surface. In another method for forming a fluorocarbon polymer thin film, the structure is exposed to a plasma environment in which a monomer gas is ionized to produce reactive CF 2  species. The plasma environment is produced by application to the monomer gas of plasma excitation power characterized by an excitation duty cycle having alternating intervals in which excitation power is applied and in which no excitation power is applied to the monomer gas. The monomer gas employed in the methods preferably includes hexafluoropropylene oxide. The monomer gas pyrolysis and plasma excitation methods can be carried out individually, sequentially, or simultaneously. Flexible fluorocarbon polymer thin films can thusly be produced on wires, twisted wires, neural probes, tubing, complex microstructures, substrates, microfabricated circuits, and other structures. The thin films have a compositional CF 2  fraction of at least about 50%, a dielectric constant of less than about 1.95, and a crosslinking density of less than about 35%.

GOVERNMENT RIGHTS IN THE INVENTION

This invention was made with U.S. Government support under ContractNumber NOI-NS-3-2301, awarded by the National Institutes of Health, andunder Contract Number F19628-95C-0002, awarded by the Air Force. TheU.S. government has certain rights in this invention.

FIELD OF THE INVENTION

This invention relates to polymer thin films, and more particularlyrelates to polymer thin films having properties like that of bulkpolytetrafluoroethylene.

BACKGROUND OF THE INVENTION

Bulk polytetrafluoroethylene, also known as, e.g., PTFE, (CF₂)_(n), andTeflon™, is characterized by superior mechanical and electricalproperties that are important for a wide range of applications. Forexample, bulk PTFE is characterized by a low dielectric constant ofabout 2.1 and a low dielectric loss factor of less than about 0.0003between about 60 Hz and 30,000 MHz. Bulk PTFE is also characterized byhigh chemical stability, exemplified by its immunity to even strongalkalis and boiling hydrofluoric acid; low water absorption, exemplifiedby its water uptake of only about 0.005 weight % in a 24 hour period;and high thermal stability, exemplified by its weight loss of only about0.05 weight % per hour at about 400° C. A low coefficient of friction ofbetween about 0.05-0.08 and a low permeability constant alsocharacterize bulk PTFE. Bulk PTFE is also well-accepted as asubstantially bio-compatible material that is tolerable by biologicalsystems such as the human body.

Many biomedical and other applications are not optimally addressed bybulk PTFE, however. For example, biologically-implantable devices suchas neural probes, catheter inserts, implantable tubing, and other suchdevices, all of which are becoming increasingly complicated in geometry,are preferably encapsulated with a film to render the devices imperviousto a biological environment, rather than being housed in a bulky PTFEpackage structure. Such implantable devices typically require of anencapsulating film not only the desired biological compatibility, butdue to complex topology and connections to lead wires and associatedcircuitry, also inherently require an encapsulating film to be conformaland thin, as well as electrically insulating, tough, and flexible. Sucha film should further be a good permeation barrier against theimplantation environment. A bulk PTFE package structure is thus notoptimally applicable to such configurations.

The properties of bulk PTFE are also desirable for thin films employedin the area of microelectronic circuit fabrication. As the desired speedof microfabricated circuits continues to increase, insulating thin filmscharacterized by a correspondingly lower dielectric constant are neededto microfabricate circuit devices having requisite lower characteristictime constants. Additionally, as the functional complexity ofmicrofabricated circuits increases, e.g., with an increasing number ofconducting multi-layer interconnects, robust, low-dielectric insulatingfilms are needed to maintain reliable electrical isolation of themulti-layer interconnects as well as to support vias between the layers.Further, robust encapsulation barrier films for microfabricationcircuits are becoming increasingly important as circuit applicationswith harsh operating environments are developed.

There have been proposed various film deposition processes devised withthe aim of producing thin films having properties similar to that ofbulk PTFE. For example, continuous radio-frequency plasma-enhancedchemical vapor deposition techniques have been proposed for producingPTFE-like films. The films typically produced by such processes havebeen found, however, to be substantially lacking in one or more criticalproperties. In particular, the stoichiometry of the resulting filmsgenerally differs rather widely from that of bulk PTFE. A typical ratioof fluorine to carbon (F/C ratio) for these films is only about 1.6,whereas bulk PTFE is characterized by a F/C ratio of 2.0. The filmsproduced by various proposed processes are also typically characterizedby a low fraction of CF₂ groups; in contrast, bulk PTFE is composedsubstantially of CF₂ groups. The high degree of crosslinkingcorresponding to low CF₂ fractionality results in film brittleness,which is unacceptable for applications in which it is desired toencapsulate a flexible, bendable structure in a fluorocarbon film.

Furthermore, carbon- 1s X-ray photo emission spectroscopy (XPS) of filmsdeposited by various of the proposed processes reveals that in additionto requisite CF₂ groups, comparable concentrations of unwanted moietiessuch as CF₃, CF, and quaternary carbon moieties are also found in thedeposited films. Also, unlike bulk PTFE, films produced by the proposedprocesses typically contain carbon-carbon double bonds and furthercontain a significant concentration of dangling bonds. The unpairedelectrons of these dangling bonds can be present in concentrations ashigh as about 10¹⁸ -10²⁰ spins/cm³, and result in highly reactive filmsurface sites.

Taken together, these various unwanted moieties and defects present intypical encapsulating films result in film properties that are sharplydegraded below that of bulk PTFE. For example, the dangling bonds in thefilms, being reactive sites, can react with atmospheric oxygen or water,producing aging effects that result in undesirable film propertyvariations over time. The dielectric properties of the films are alsodegraded, resulting in, e.g., an increased dielectric loss. The chemicaland thermal stability of the films are also degraded, due to thesuboptimal film stoichiometry. Thus, deposition techniques devised in aneffort to duplicate properties of bulk PTFE for thin-film applicationshave generally resulted in only suboptimal thin films that typicallycannot adequately address performance requirements of PTFE thin filmapplications.

SUMMARY OF THE INVENTION

The invention overcomes limitations of prior deposition processes toenable production of fluorocarbon polymer thin films having materialsproperties of bulk PTFE, and addresses the many biomedical andmicrofabrication applications for such a film. Accordingly, in oneaspect, the invention provides a method for forming a fluorocarbonpolymer thin film on the surface of a structure. This is accomplished byexposing a monomer gas to a source of heat having a temperaturesufficient to pyrolyze the monomer gas and produce a source of reactiveCF₂ species in the vicinity of the structure surface. The structuresurface is maintained substantially at a temperature lower than that ofthe heat source to induce deposition and polymerization of the CF₂species on the structure surface.

Preferably, the monomer gas includes hexafluoropropylene oxide, and theheat source preferably is a resistively-heated conducting filamentsuspended over the structure surface or a heated plate having apyrolysis surface that faces the structure. The heat source temperatureis preferably greater than about 500K and the structure surface ispreferably substantially maintained at a temperature less than about300K.

The structure on which surface the film is formed can be, in exemplarylo embodiments, a length of wire, a substrate, a neural probe, a razorblade, or a microstructure having multiple surfaces all maintainedsubstantially at a temperature lower than that of the heat source.

In other embodiments, a first step of applying plasma excitation powerto the monomer gas is carried out, and a last step of applying plasmaexcitation power to the monomer gas is carried out. In an exemplaryembodiment, the monomer gas is not substantially pyrolyzed during plasmaexcitation power application. In other embodiments, the monomer gas isexposed to the heat source simultaneously with application of plasmaexcitation power to the monomer gas. Whenever plasma excitation power isapplied, it preferably is characterized by an excitation duty cyclehaving alternating intervals in which excitation power is applied and inwhich no excitation power is applied to the monomer gas.

In another aspect, the invention provides a method for coating a nonplanar and flexible structure with a flexible fluorocarbon polymer film.The coating is accomplished by exposing the structure to a plasmaenvironment in which a monomer gas is ionized to produce reactive CF₂species. The plasma environment is produced by application to themonomer gas of plasma excitation power characterized by an excitationduty cycle having alternating intervals in which excitation power isapplied and in which no excitation power is applied to the monomer gas;the monomer gas preferably includes hexafluoropropylene oxide.

Preferably, the interval of the plasma excitation power duty cycle inwhich excitation power is applied is between about 100 microseconds and0.1 seconds, and more preferably between about 1 millisecond and 100milliseconds, and the interval of the plasma excitation power duty cyclein which no excitation power is applied is preferably between about 100microseconds and 1 second, and more preferably between about 350milliseconds and 450 milliseconds. The plasma excitation preferablyprovides a power of between about 100 and 300 Watts, with the plasmaenvironment being produced at a pressure of between about 1 milliTorrand 10 Torr.

The invention also provides a method for substantially encapsulating alength of wire in a flexible fluorocarbon polymer thin film. In themethod, the wire length is supported on a wire holder such that surfacesof the wire length are substantially unmasked and portions of the wirelength are out of contact with each other. The wire length is exposed toa plasma environment in which a monomer gas is ionized to producereactive CF₂ species, with the plasma environment being produced byapplication to the monomer gas of plasma excitation power characterizedby an excitation duty cycle having alternating intervals in whichexcitation power is applied and in which no excitation power is appliedto the monomer gas.

In embodiments provided by the invention, the encapsulation process alsoincludes the step of exposing the monomer gas to a heat source topyrolyze the monomer gas and produce a source of reactive CF₂ species inthe vicinity of the wire length, which is, e.g., between about 10microns and 100 microns in diameter. The wire length is substantiallymaintained at a temperature lower than that of the heat source to inducedeposition and polymerization of the CF₂ species on the wire length.

In another aspect of the invention, there is provided a method forcasting a flexible structure in a desired configuration. This isaccomplished by deforming the structure into the desired configurationand exposing the deformed structure to a plasma environment in which amonomer gas is ionized to produce reactive CF₂ species. The plasmaenvironment is here produced by application to the monomer gas of plasmaexcitation power characterized by an excitation duty cycle havingalternating intervals in which excitation power is applied and in whichno excitation power is applied to the monomer gas. Exposure of thedeformed structure to the plasma environment is maintained for aduration sufficient to produce on the deformed structure a fluorocarbonpolymer film having a thickness of more than about 5 microns.

In other aspects, the invention provides a length of wire that includesa wire core and a flexible fluorocarbon polymer thin film encapsulatingthe wire core along at least a portion of the wire length. The polymerthin film has a compositional CF₂ fraction of at least about 50%, adielectric constant of less than about 1.95, and a crosslinking densityof less than about 35%. Preferably, the compositional CF₂ fraction ofthe film is at least about 60%. In one embodiment, the wire core has adiameter of between about 10 microns and 100 microns and the film has athickness of between about 0.1 microns and 100 microns. Preferably, thepolymer thin film has a fluorine to carbon ratio of greater than about1.8.

The invention also provides a length of twisted wire that includes aplurality of entwined wires and a flexible fluorocarbon polymer thinfilm encapsulating the entwined wire plurality along at least a portionof the twisted wire length. Also provided by the invention is a lengthof tubing that includes a thin-walled, flexible polymeric cylinder andflexible fluorocarbon polymer thin film on an outer surface of thecylinder along at least a portion of the tubing length. The polymer thinfilm provided in these embodiments preferably has a compositional CF₂fraction of at least about 50%, a dielectric constant of less than about1.95, and a crosslinking density of less than about 35%.

In another aspect, there is provided by the invention a neural probe.The neural probe includes a substantially cylindrical shaft portionhaving a diameter less than about 100 microns and a tip portionconnected to the shaft portion by a tapered shaft portion. The tip has adiameter less than about 15 microns. A flexible fluorocarbon polymerthin film encapsulates the tapered shaft portion and cylindrical shaftportion of the probe. The polymer thin film has a compositional CF₂fraction of at least about 50%, a dielectric constant less than about1.95, and a crosslinking density less than about 35%. Preferably, thecompositional CF₂ fraction is at least about 60% and the polymer thinfilm is between about 1 micron and 20 microns in thickness.

The invention additionally provides a substrate having a fluorocarbonpolymer thin film of less than about 20 microns in thickness thereon;and further provides a microfabricated electronic circuit including atleast one conducting layer having a fluorocarbon polymer thin film ofless than about 20 microns in thickness thereon. In embodiments providedby the invention, the compositional CF₂ fraction of the polymer thinfilm on the substrate or conducting layer is at least about 70%, and thedielectric constant is less than about 1.95.

In exemplary embodiments, the polymer thin film on the substrate orconducting layer has a dangling bond density less than about 10¹⁸spins/cm³, a crosslinking density less than about 15%, a dielectricconstant less than about 1.8, and preferably has a compositional CF₂fraction of at least about 85%.

The various methods of forming fluorocarbon polymer thin films providedby the invention, and the resulting films, address a wide range of thinfilm applications for bulk PTFE, including biomedical andmicrofabrication applications, as well as numerous mechanicalconfigurations in which a thin coating of PTFE-like material is desired.Other features and advantages of the invention will be apparent from theclaims, and from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a vacuum chamber apparatus suitable forcarrying out the film deposition processes provided by the invention;

FIG. 2A and FIG. 2B are side-view and top-view schematic diagrams,respectively, of a support holder for supporting a length of wire to beprocessed in accordance with the invention in the vacuum chamber of FIG.1;

FIGS. 3A, 3B, and 3C are plots of the carbon-1s X-ray photo emissionspectra for a fluorocarbon polymer thin film produced by a continuousplasma-enhanced chemical vapor deposition process in the prior art, fora fluorocarbon polymer thin film produced by a pulsed-plasma-enhancedchemical vapor deposition process in accordance with the invention, andfor bulk polytetrafluoroethylene, respectively;

FIGS. 4A and 4B are environmental scanning electron microscopy views ofloops of wires coated with the films characterized by the spectral plotsof FIGS. 3A and FIGS. 3B, respectively;

FIG. 4C is an environmental scanning electron microscopy view of across-sectional cut through the wire loop of FIG. 4B;

FIG. 5 is an environmental scanning electron microscopy view of a neuralprobe coated with a fluorocarbon polymer film produced by apulsed-plasma-enhanced chemical vapor deposition process in accordancewith the invention;

FIG. 6 is a side-view schematic diagram of a hot-wire filament apparatusfor use with the vacuum chamber apparatus of FIG. 1 in a thermalchemical vapor deposition process in accordance with the invention;

FIG. 7 is a plot of the carbon-1s X-ray photo emission spectra for afluorocarbon polymer thin film produced by a thermal chemical vapordeposition process in accordance with the invention; and

FIG. 8 is an environmental scanning electron microscopy view of across-sectional cut through a wire coated with a thin film produced by athermal chemical vapor deposition process in accordance with theinvention.

DETAILED DESCRIPTION

The thin film deposition processes provided by the invention enabletailoring of the chemical composition of deposited films to producefluorocarbon polymer thin films having stoichiometry and materialsproperties similar to that of bulk PTFE. The thin films resulting fromthe processes of the invention have superior materials properties overprior thin films, which generally fail to match the materials propertiesof bulk PTFE.

In a first deposition process in accordance with the invention, astructure to be coated with a PTFE-like thin film is exposed to afluorocarbon monomer species under pulsed-plasma-enhanced chemical vapordeposition conditions (pulsed PECVD conditions). An rf plasma depositionsystem like that schematically illustrated in FIG. 1 can be employed forcarrying out the deposition process. As will be recognized by thoseskilled in the art, other conventional plasma deposition systems canalternatively be employed. The example deposition system 10 includes anair-tight vacuum chamber 12 formed of, e.g., steel, and includes apowered electrode 14 and a ground electrode 16 each formed of, e.g.,aluminum.

The powered electrode 14 is preferably configured with connection to afeed gas source 18 such that the gas 20 is introduced into the chamber,e.g., through tubes in the powered electrode in a conventionalshower-head configuration. Preferably, the shower-head tubes provide areasonably equal flow of gas per unit area of the upper electrode.Accordingly, the shower-head tubes should be spaced such that theconcentration of the gas injected out of the shower-head is relativelyuniform. The number and spacing of the tubes is dependent upon thespecific pressure, electrode gap spacing, temperature, and other processparameters, as will be recognized by those skilled in the art. Forexample, for a typical process employing a pressure of about 1 Torr andan electrode gap spacing of about 1 cm, the shower-head tube spacing isabout 1 cm.

A flow rate controller 22 is preferably provided to enable control ofthe flow of gas through the powered electrode into the chamber. Thepowered electrode is also connected electrically to a radio frequency(rf) power source 24, or other suitable power source, for producing aplasma of the feed gas in the chamber.

The grounded electrode 16 is connected electrically to a ground 26 ofthe vacuum chamber system. Preferably, the grounded electrode 16provides a surface 28 for supporting a substrate or other structure ontowhich a thin film is to be deposited. The grounded electrode and itssupport surface are preferably cooled by way of a cooling systemincluding, e.g., a coolant loop 30 connected to cooling coils 31 and atemperature controller 32, enabling a user to set and maintain a desiredelectrode temperature by way of, e.g., water cooling.

A pump 34 is provided for evacuating the deposition chamber to a desiredpressure; the pressure of the chamber is monitored by way of, e.g., apressure gauge 36. Also preferably provided is an analysis port 36 forenabling a user to monitor progress of the deposition process.

The pulsed PECVD film deposition process of the invention, as describedbelow, produces a flexible, conformal fluorocarbon coating that can beapplied to a wide range of structures including complexthree-dimensional geometries. Planar substrates, e.g., conventionalmicrofabrication wafer substrates, or other planar structures, can alsobe singly or batch processed. In a single-substrate process, thesubstrate is supported on the grounded electrode 28. In amulti-substrate process, a plurality of substrates can be suspended inthe vacuum chamber between the powered and grounded electrode by way of,e.g., an aluminum substrate boat, vertically supported by, e.g., achamber sidewall anchor, and having support slots for holding substratesin a desired configuration. Preferably, the selected multi-substratesupport configuration enables a user to adjust individual substrates'positions without substantial complexity; such substrate positionadjustment may be desirable at intervals during a deposition process forenhancing deposition uniformity across the span of a substrate.

Deposition of a fluorocarbon polymer thin film provided by the inventioncan also be carried out on cylindrical objects such as thin cylindricalstructures. For example, interconnection wires for integrated circuits,lead wires for pacemakers and other biomedical devices, and in general,any wiring structure for which a PTFE-like coating is desired, can becoated by the deposition process provided by the invention. Importantly,because the deposition conditions, as outlined below, enableroom-temperature deposition, many wiring materials can be accommodatedby the process. For example, single-stranded stainless steel or copperwire, or twisted groups wires such as twisted filler wires used inpace-maker leads, can be accommodated by the process.

Referring to FIGS. 2A and 2B, the invention provides a wire holder forsupporting wire on which a PTFE-like coating is to be deposited in thedeposition chamber. In one example configuration, the wire holderincludes a holding ring 40, e.g., an aluminum ring, with a peripheraledge ridge 41. The holding ring and edge ridge geometry preferablycorrespond to the shape of the support surface 28 of the groundedelectrode 16. With this geometry, the holding ring can be mated with thegrounded electrode. As shown in both the cross-sectional view of FIG. 2Aand the top-down view of FIG. 2B, the holding ring 40 includes posts 44,e.g., aluminum posts, at points around the circumference of the ring,for suspending a length of wire 42 above the grounded electrode 16;preferably, the wire is suspended about 0.5 cm above the groundedelectrode. A length of wire to be coated is accordingly wrapped one ormore times between the posts at a point on the posts above the holdingring surface preferably such that all sides of the wire are accessibleto the deposition plasma between the powered electrode 14 and thegrounded electrode 16, and preferably such that no two or more windingsor the wire are in contact. One or more screws 46, or other fasteningdevices, are preferably provided for fastening the length of wire ontothe holding ring.

Multiple lengths of wire can be individually fastened to and supportedby the holding ring configuration for simultaneous coating of the wires.During the coating process, the one or more lengths of wire can berotated, but as will be recognized by those skilled in the art, suchrotation is not required. Indeed, the nature of the plasma environmentproduced by the processes of the invention substantially entirelyimmerses the wires in the plasma. Rotation of the wires can be employedfor enhancing the uniformity of the wire coating. In one examplerotation technique, the wires are rotated about their longitudinal axis,by, e.g., manually adjusting the wires. In a second example rotationtechnique, the wires are rotated around points in the depositionchamber. In this case, the holding ring can be configured, e.g., to spinlike a record on a turntable such that portions of the wire lengths areperiodically moved around the deposition chamber.

As will be recognized by those skilled in the art, various other supportstructures can be employed to accommodate a cylindrical structure duringthe deposition process. For example, in the case of coating of a longcontinuous length of wire, take-off and takeup spools can be provided toenable a continuous coating operation. Here the spools are preferablycontrollable such that the wire length is drawn through the depositionplasma at selected intervals corresponding to a desired coatingthickness, at a continuous rate, or other desired control scheme.

Structures having geometry other than cylindrical and having a widerange of topology are also accommodated by the deposition process of theinvention. For example, catheter inserts, neural probes, tubing, shaftstructures, and other three-dimensional structures having multiplesurfaces can be accommodated. For example, neural probe shaft structureslike, e.g., iridium probes, having a cylindrical shaft that tapers to atip, can be coated by the deposition process provided by the invention.The invention provides a probe holder for supporting one or more probesduring the deposition process. One example probe holder includes a PTFEblock having one or more holes drilled in the block to a selected depthsuch that a probe can be supported in each hole. During the depositionprocess, the PTFE block is supported on the grounded electrode in thedeposition chamber; the hole depths are accordingly selected such thatthe probes are held a desired distance above the grounded electrode. Inan example process, the probes are preferably held about 1.0 cm abovethe grounded electrode.

As mentioned above, the processing conditions of the deposition processenable room-temperature fluorocarbon film deposition. Thus, heatsensitive materials, such as organic polymers like urethanes, can beaccommodated by the process. Polymer structures such a polymeric tubing,and other polymer structures can therefore be coated. Low-temperaturedeposition conditions are also desirable for coating structures thatinclude interfaces of two or more materials that characteristicallyinter diffuse, or that have different thermal expansion coefficients atrelatively higher temperatures. Also, structures containing activebiological components, such as enzymes, pharmaceuticals, or live cells,may in many applications be best processed at relatively lowerdeposition temperatures.

Other than polymer tubing, any tubing structure can be coated by theprocess provided by the invention. The deposition processes provided bythe invention are especially well-suited for deposition on thin-walledtubing, e.g., tubing having a wall thickness of between about 1/128inches and 1/4 inches. In one example, a coating is deposited on theexterior of the tube structure along its length such that during use ofthe tube to carry gases or liquids, the gases and liquids cannotpermeate through the tube wall. In one example deposition process, alength of tubing to be coated is slid over a corresponding length ofwire, which in turn is supported on the holding ring shown in FIG. 2.Preferably, the various considerations described above in connectionwith wire coating are also accounted for in this case. In a secondexample, one or more lengths of tubing are vertically suspended in thedeposition chamber, without the need for an internal wire support. Aswill be recognized by those skilled in the art, other tubing supporttechniques are also suitable.

Other complex geometries, e.g., tubing having stabilizing fins, andother three-dimensional structures, can also be accommodated. As will berecognized by those skilled in the art, a support structure, e.g., abulk PTFE block, or other support structure, can be employed toaccommodate a specific object to be coated in the deposition chamber. Inaddition, reorientation of the object to be coated and its supportstructure can be enabled by, e.g., manual reorientation during thedeposition process, or like the spinning wire holder ring describedabove, can be designed-in as a mechanism integral to the supportstructure. Substrate and object reorientation techniques, as areroutinely employed in industrial vapor deposition and ion-implantationprocesses, can correspondingly be employed in the invention to enhancedeposition uniformity.

Turning now to the processing conditions prescribed by the pulsed-PECVDdeposition process of the invention, it is well-recognized that manycomplex physical phenomena and interactions occur in any chemical vapordeposition (CVD) environment. For PECVD, experimentally controllableprocess parameters that affect plasma electron density, electron energydistribution, gas molecule in-chamber residence time, and gas density,as well as process gas flow rate, feed gas composition, plasmaexcitation frequency, excitation power, chamber pressure, and chamberreactor geometry all directly affect the chemical processes that occurduring a PECVD process. Further, the surface geometry of a structure tobe coated, as well as the chemical composition of the structure, thestructure's temperature, and the structure's electrical potential allaffect the nature of the plasma-surface interactions that occur during aPECVD process. Thus, as will be recognized by those skilled in the art,the various process parameters can be adjusted over a wide range toachieve any of a continuum of deposition process conditions. Preferably,the various parameters are controlled such that the deposition processis optimized for a given structure geometry, composition, andapplication.

Whatever process parameters are selected, an initial adhesion-promotionstep can be employed prior to the deposition process to enhance andpromote adhesion of the depositing species on a given structure. Forexample, an adhesion promoter can be spin-applied to a planar substrateor sprayed on to a complex geometrical object. Alternatively, anadhesion promoter can be vapor-deposited in situ in the depositionchamber just prior to the fluorocarbon polymer film deposition process.Examples of suitable adhesion promoters include 1H, 1H, 2H,2H--Perfluorodecyltriethoxysilane; 1H, 1H, 2H, 2H -Perfluorooctyltriethoxysilane; 1H, 1H, 2H,2H--Perfluoroalkyltriethoxysilane; perfluorooctyltriclorosilane; allclasses of vinyl silanes, as well as other adhesion promoters, as willbe recognized by those skilled in the art.

In the pulsed-PECVD deposition process provided by the invention, aplasma is provided in the deposition chamber by pulsing the rfexcitation, or other plasma excitation, applied to the feed gasintroduced into the chamber. In other words, the rf excitation power isalternately turned on and off with a desired duty cycle, rather thanbeing applied as a continuous plasma excitation. This pulsed-plasmaexcitation technique enables a much wider range of process control thanis typical of conventional, continuous-plasma deposition processes. Inthe invention, the plasma excitation on-time is between about 1microsecond and 1 second, and the plasma excitation off-time is betweenabout 1 microsecond and 10 seconds. Preferably, to deposit PTFE-likepolymer thin films, the plasma excitation on-time is about 10milliseconds and the plasma excitation off-time is about 400milliseconds.

During the plasma excitation on-time, ions and reactive neutral speciesare produced in the feed gas introduced between the electrodes in thedeposition chamber. During the plasma excitation off-time, no additionalions or reactive neutral species are produced, and the ions producedduring the on-time decay more quickly than the reactive neutral speciesproduced during the on-time. As a result, during the on-time, reactivespecies deposit and polymerize on the structure and ions bombard thestructure, while during the off-time, no significant ion bombardmentoccurs. As is well-recognized, PECVD processes fundamentally consist ofa competing deposition ablation processes as a result of ion bombardmentduring the deposition. It has been found that with the pulsed-PECVDprocess of the invention, reasonable film deposition rates aremaintained even at small plasma excitation on/off-time ratios. Thisindicates that the reactive neutral species produced during the plasmaexcitation on-time of each pulse duty cycle likely survive sufficientlylong during the off-time of the pulse duty cycle such that the filmgrowth reaction favors deposition during this time. Thus, thepulsed-PECVD process reduces the degree of ion bombardment of astructure being coated while substantially maintaining reasonabledeposition rates.

Ion bombardment is recognized as a principal factor causing damage to asurface on which a film is being deposited, e.g., by causing danglingbond formation. Indeed, it has been found that plasma exposure can evendamage the surface of bulk PTFE, producing both significantdefluorination and increased crosslinking of the bulk surface, bothconditions resulting in degradation of the desired PTFE materialsproperties. Thin films produced by a continuous-plasma PECVD process arefound to exhibit the same distribution of quaternary carbon, CF, CF₂,and CF₃ moieties as that produced by etching and/or ablation of bulkPTFE by plasma exposure. It is thus recognized that an optimaldeposition process for producing thin films having properties resemblingbulk PTFE reduces or suppresses deleterious effects due to plasma ionbombardment of the depositing film.

Considering the range of plasma-excitation duty cycles employed in theinvention, it has been found that the film deposition rate, filmcompositional fraction of CF₂ groups, and film dangling bondconcentration each increase and then plateau as the plasma excitationoff-time is increased about 400 milliseconds, with the plasma excitationon-time held fixed at about 10 milliseconds. Films resulting from anoff-time of about 400 milliseconds found to be characterized by a highcompositional CF₂ fraction, as well as a low dangling bondconcentration. Variation of the feed gas flow rate, resulting in acorresponding variation of gas residence time, is found to not alterthese results. It is found that this on-off-time ratio results in adeposition rate of about 2 Å per pulse duty cycle, corresponding to thedeposition of a full monolayer per cycle. These observations, along withthe apparent negative activation energy of the process, suggest thatabsorption plays a key role in the kinetic mechanism of the pulsed-PECVDdeposition process. A fuller discussion of film characteristics will bepresented in Example 1 below.

As explained above, it has been found that continuous plasma etching,ablation, and deposition processes result in a distribution of CF_(X)species; only CF₂ groups are desired for producing a PTFE-like film,however. The pulsed-PECVD process of the invention enables the abilityto produce a plasma gas of predominantly CF₂ species, thereby reducingthe fraction of other moieties in the resulting film. The feed gasemployed in the deposition process of the invention is also selected tomaximize the CF₂ reaction species. In particular, the feed gas isselected such that the plasma gas phase decomposition product ispredominantly CF₂. Example monomers for use as a deposition feed gasinclude C₂ F₄, C₃ F₈, CF₃ H, CF₂ H₂, CF₂ N₂ (difluordiaxirine), CF₃COCF₃, CF₂ ClCOCF₂ Cl, CF₂ ClCOCFCl₂, CF₃ COOH, difluorohalomethanessuch as CF₂ Br₂, CF₂ HBr, CF₂ HCl, CF₂ Cl₂, and CF₂ FCl;difluorocyclopropanes such as C₃ F₆, C₃ F₄ H₂, C₃ F₂ Cl₄, C₂ F₃ Cl₃, andC₃ F₄ Cl₂ ; trifluoromethylfluorophosphanes such as (CF₃)₃ PF3, (CF₃)₂PF₃, and (CF₃)PF₄ ; or trifluoromethylphosphino compounds such as (CF₃)₃P, (CF₃)₂ P--P(CF₃)₂, (CF₃)₂ PX, and CF₃ PX₂, where X is F, Cl, or H.Other monomers can also be employed.

One preferable monomer is hexafluoropropylene oxide (C₃ F₆ O or HFPO).HFPO is characterized by a highly-strained epoxide ring that enableseasy ring-opening reactions with nucleophiles. It has been found thatfilms deposited using HFPO under pulsed PECVD conditions result inpolymer films having a high CF₂ fraction and little or no oxygenincorporation. This suggests that neutral reactive species in the plasmaproduced using this monomer do survive for some time during the plasmaexcitation off-time of the pulse duty cycle, thereby enablingsignificant deposition during the off-time, when plasma ion bombardmentis significantly reduced. It has been found that films deposited usingHFPO under continuous-plasma PECVD conditions exhibit materialsproperties inferior to those produced under pulsed-plasma PECVDconditions; thus, it is expected that the selected monomer andpulse-plasma excitation conditions synergistically provide the desiredPTFE materials properties.

Considering the selection of gas feed monomer in general, it isrecognized that the ratio of CF_(X) /F in the gas directly effects thecompeting deposition and etching reactions that occur during aplasma-based deposition process; a higher ratio corresponds toenhancement of deposition and suppression of etching reactions. It hasbeen found that this ratio can be increased by including in a feed gascomposition a fluorine scavenger, e.g., hydrogen, a hydrocarbon, or anunsaturate. In general, the addition of hydrogen or C₂ F₄ to afluorocarbon feed gas is found to result in decreasing atomic Fconcentration relative to CF_(X) concentration. This decreased atomic Fconcentration correspondingly results in increased deposition rate.Additionally, the inclusion of hydrogen in the feed gas can alter thegap-filling capabilities of the deposited film due to its reduction inion bombardment. Furthermore, hydrogen can be included in a feed gas toprovide an in situ mechanism for passivating dangling bonds on thesurface of a structure being processed. For example, it is known thathydrogen can passivate amorphous silicon dangling bonds.

The selection of feed gas constituents should also preferably take intoconsideration any trace impurities that could resultingly beincorporated into a film deposited from the feed gas. For example, HFPOas a feed gas monomer can result in incorporation of trace amount ofoxygen in a deposited film. Thus, if trace oxygen is not acceptable fora deposited film, a feed gas monomer other than HFPO is preferable.Other process parameters should likewise preferably be considered inselecting a feed gas monomer, as will be recognized by those skilled inthe art.

Turning to other parameters of the pulsed-PECVD process provided by theinvention, the rf plasma excitation power ranges between abut 50 to 280Watts for a 4.5 inch diameter grounded electrode surface, with the powerpreferably being about 280 Watts. For a given electrode geometry, thepreferred power density is about 2.7 Watts/cm². The rf plasma excitationfrequency is set at, e.g., about 13.56 MHz, as is conventional forplasma processes, but other frequencies can be employed. The flow rateof the monomer feed gas ranges between about 1 sccm and 200 sccm, forone conventional reactor geometry, with a flow rate of about 30 sccmpreferable for the monomer HFPO. Inert gases such as nitrogen or argoncan be added to the monomer feed gas; preferably, no inert gas isincluded with the monomer HFPO, however.

Variations in power and flow rate effect the plasma concentration andcorrespondingly effect gas residence time and film deposition rate.Increase in plasma excitation power increases electron densities, anddirectly enhances the tail of the electron energy distribution function.Also, it has been found that the atomic concentration of fluorinedramatically increases as power is increased, e.g., for a gas consistingof C₂ F₆ and 20% H₂. The CF₂ concentration is also found to increasewith increasing power to a certain power level, above which it decreasesdue to recombination of the CF₂ with the increased fluorine. Monomerfeed gases including hydrogen, however, result in a lower fluorine atomconcentration, and therefore result in processes that are less sensitiveto power variations. In this case, the deposition rate is found togenerally increase with increasing power due to a high concentration ofactive deposition precursor.

The structure onto which a PTFE-like film is to be deposited can be heldat an electrical potential of between about -400 V to +400 V, includinga ground potential or floating potential at the potential of the plasma.Ground potential or floating potential are preferred. In general, theenergy with which plasma ions bombard the structure's surface decreaseswith decreasing structure potential, relative to the plasma potential.This energy variability impacts the film deposition rate and thechemical composition of the film. It also affects the film topologicalcoverage.

In the invention, recognition of this energy-dependent ion bombardmentis purposefully employed to enable in situ self masking of a structureon which a film is being deposited. In one example application, eitherthe entire surface or only selected surface regions of a neural probeare coated with a polymer film without the use of masking material. Aneural probe, as described earlier, consists of a cylindrical shaftwhich at one end is tapered to a point. If the probe, supported in,e.g., a PTFE block on the lower chamber electrode, is held at a floatingpotential, whereby it equilibrates to the potential of the plasma, theentire surface of the probe, including the shaft and tip, are coatedwith the depositing film. In contrast, if the probe is maintained at theground potential of the grounded electrode, the typically very narrowtip undergoes substantial ion bombardment during the deposition, whilethe shaft does not. This results in a net film deposition on the shaftand substantially no film deposition on the probe tip. Typical neuralprobe applications require this very configuration; i.e., the tip of theprobe preferably is uncoated such that electrical contact to neuraltissue is accomplished, but the shaft of the probe is preferably coatedsuch that its biocompatibility is maximized. Thus, the inventionprovides the ability to produce an effective masking of selected regionsof a structure based on the geometry of the structure and the potentialat which the structure is held during the deposition. As will berecognized by those skilled in the art, this example can be extended toother geometries and potentials for producing in situ masking ofstructural surfaces during the deposition process.

The pressure of the vacuum deposition chamber can be set at a pressureof between about 1 milliTorr to 50 Torr during the deposition process,with a pressure of about 1 Torr being preferable. Pressure variationscan result in large changes in plasma species concentrations. Pressureimpacts gas residence time directly, impacts average electron energyinversely, and impacts mean free path of radical species proportionally.Generally, a low mean free path, long residence time, and relativelyhigh electron energy produce an environment favorable for depositionconditions. An increase in pressure has been found to result in anincrease in CF_(X) reactive species concentrations, including CF₂concentrations, which as explained earlier is desirable for producingPTFE-like films. The increased reactive species concentration results incorrespondingly higher deposition rates. Furthermore, the maximumdeposition rate per plasma excitation duty cycle has been found toincrease with pressure, all other parameters held constant.

The structure onto which a film is being deposited is held at atemperature of between about -40° C. and +200° C. during the deposition;preferably the temperature is held to about 293K. The temperature thatis maintained during film deposition can be an important factor fordetermining the ultimate thermal stability of a film produced by thedeposition process. Films deposited at relatively higher structuraltemperatures may in some applications be relatively more resistant toheating. This is a critical property for films to be employed, e.g., asinterlayer dielectric materials in microfabrication processes; suchfilms preferably can withstand thermal treatments associated withmetalization processes subsequent to their deposition.

The upper powered electrode and lower grounded electrode are preferablyspaced apart by, e.g., about 0.25 inches to about 12 inches, dependingon the reactor configuration. In the example process described here, theelectrode spacing is preferably about 1 inch.

In the deposition process provided by the invention, thermalpost-deposition steps can be carried out in situ in the depositionchamber. For example, a post-deposition annealing in air, or in nitrogenor other inert gas can be employed for, e.g., film stress relief,dangling bond passivation, or thermal stability enhancement. Suchannealing can be carried out at a temperature of between about 50° C.and 400° C.

Fluorocarbon polymer thin films deposited pursuant to the invention arecharacterized as smooth, conformal, coatings that exhibit sufficientflexibility to withstand mechanical bending of a three-dimensionalstructure, e.g., a wire, on which they are deposited. The films exhibitmaterials properties that closely resemble those of bulk PTFE, therebyenabling a wide range of thin film applications that heretofore havebeen met with only suboptimal results.

In particular, the fluorine to carbon ratio (F/C ratio) of filmsprovided by the invention is preferably between about 1.1:1-2.2:1; thisfluorine-rich composition results in many of the well-known bulk PTFEproperties. The CF₂ fraction of films provided by the invention aregreater than about 50%, and preferably greater than about 60%. Also, thedangling bond density of films provided by the invention is very low,preferably being less than about 10¹⁸ /cm³. The films are alsocharacterized by a low polymer crosslinking density of less than about35%, and preferably less than about 18%. This results in increasedflexibility, which in turn provides film stress relief and enables thefilms to withstand physical handling and exposure to environments suchas biological environments. Films provided by the invention are alsocharacterized by a dielectric constant of between about 1.4 and 1.95.This is much lower than that of previous films attempting to duplicatethe dielectric properties of bulk PTFE.

EXAMPLE 1

A fluorocarbon polymer thin film was produced in accordance with theinvention by flowing about 25 sccm of undiluted HFPO (from PCR, Inc.) ofabout 99% purity, into a parallel plate vacuum deposition chamber likethat described above. The volume between the upper powered electrode andthe lower grounded electrode was about 261 cm³. The reactor was pumpedto a pressure of about 1 Torr, and the lower grounded electrode wascooled to maintain it at a temperature of about 293±3K by way ofbackside water cooling. An aluminum holding ring was employed on thegrounded electrode to support several lengths of stainless steel wireeach of about 25 μm in diameter. The ends of the wires were electricallygrounded to the electrode and were maintained in thermal contact withthe electrode. The wire lengths, as supported in the holding ring, wereheld about 0.5 cm above the lower grounded electrode, which wasseparated from the upper powered electrode by about 1 inch. Films weredeposited on the wires by exciting the HFPO feed gas by application of apulsed plasma excitation. The rf power density was about 2.7 W/cm² andthe rf frequency was about 13.56 MHz. The pulsed plasma excitation dutycycle consisted of a plasma excitation on-time of about 10 ms and aplasma excitation off-time of about 400 ms.

For comparison, a continuous plasma CVD process was also carried out onlengths of stainless steel wire supported in a holding ring on a lowergrounded electrode. All process parameters were maintained identical tothat of the pulsed-PECVD parameters except for the rf power density; apower density of about 0.49 W/cm² was employed for the continuous plasmaprocess because it is known that at higher power densities, etching,rather than deposition, occurs.

The pulsed-PECVD process and continuous-PECVD processes were eachcarried out to produce a fluorocarbon coating of a thickness of about 10μm on the stainless steel wires. A chemical compositional analysis ofthe coatings is shown in FIGS. 3A and 3B. The figures show plots of thecarbon-1s X-ray photoelectron spectra (XPS spectra) for the films as afunction of binding energy. For the continuous-PECVD film, the spectrumpeak at 292 eV, as shown in FIG. 3A, is indicative of a CF₂ bondingenvironment. However, deconvolution of the spectrum reveals that the CF₂peak accounts for only about 21% of the spectrum's area, and that theother peaks represent CF₃, CF, and C-CF moieties, as shown in the plot.Based on measurement of the areas of these peaks, the F/C ratio of thecontinuous-PECVD film is given as only about 1.36.

In dramatic contrast, as shown in the plot of FIG. 3B, the pulsed-PECVDprocess provided by the invention resulted in a film having a CF₂fraction of about 65%. This CF₂ fraction corresponds to an F/C ratio ofabout 1.9. FIG. 3C provides the Carbon-1s X-ray spectrum for bulk PTFEfor reference. Note the very close similarity between the spectrum forthe film produced by pulsed-PECVD and that of bulk PTFE. Thisquantitatively demonstrates that the pulsed-PECVD process produces afilm having materials properties that much more closely resemble that ofbulk PTFE. Also note that the oxygen content of the PECVD films producedusing the monomer HFPO was less than about 2 atomic %. Some or all ofthis oxygen constituency may result from the films' exposure toatmospheric conditions, rather than processing conditions.

The fraction of crosslinks in the two films was determined based ondeconvolution of the XPS spectra of the films. In this determination,the number of network-forming bonds of the XPS-resolvable CF₃, CF₂, CF,and C-CF groups are taken to be 1, 2, 3, and 4, respectively. Thisassignment assumes that the number of carboncarbon double bonds issmall. Of these four resolvable groups, only the groups having more than2 bonds are characterized as cross-linkable, i.e., able to form anetwork; thus, the CF and C-CF groups are characterized as crosslinks.Accordingly, the compositional crosslinking percentage of a given filmcan be determined by the XPS deconvolution area of these two groupsrelative to the total area of the spectrum.

Considering first the XPS spectrum of FIG. 3A for the film producedunder continuous-PECVD conditions, it is seen that the CF and C-CFgroups make up more than 50% of the total film composition. Accordingly,the film produced under continuous-PECVD conditions is characterized bya crosslinking density of greater than about 50%, and in mostconventional continuous-PECVD processes, is likely greater than about60%.

In dramatic contrast, the XPS spectrum of FIG. 3B for the film producedin accordance with the invention under pulsed-PECVD conditions showsthat the majority of the film is composed of CF₂ groups. The CF and C-CFgroups account for only about 18% of the film composition. Thus, filmsproduced in accordance with the invention are composed of dramaticallyfewer crosslinking bonds than conventional films, whereby films pursuantto the invention are characterized by a correspondingly dramaticincrease in flexibility over conventional films.

In an effort to compare the structural integrity of the film producedunder pulsed-PECVD conditions with that of the film produced undercontinuous-PECVD conditions, the 25 μm diameter wires having 10 μmcoatings were tied into loops of 800 μm in diameter. Environmentalscanning electron microscopy (ESEM) was employed to examine thecondition of the tied wire loops.

FIG. 4A shows an ESEM view of the tied wire loop coated with a filmproduced by continuous-PECVD conditions. As shown in the view, the filmshattered and actually fell apart as the loop was tied. FIG. 4B shows anESEM view of the tied wire loop coated with a film produced bypulsed-PECVD conditions. The dramatic improvement in structuralintegrity is clear from the figure. In this case, the film showed noindication of structural failure; no cracks or peeling were identifiedat any point along the entire loop; furthermore, no shattering of thefilm was exhibited at any point. This experiment exemplified that thefilm produced by pulsed-PECVD conditions proved to be flexible, whilethe film produced by continuous-PECVD conditions proved to be verybrittle. The experiment thus demonstrates with particular clarity thatPTFE-like films produced pursuant to the invention, unlike priorfluorocarbon polymer films, have sufficient flexibility to withstandmechanical bending of a structure on which they are coated. Thisproperty enables the heretofore unachievable ability to encapsulateflexible structures with a fluorocarbon polymer thin film having thedesired materials properties of bulk PTFE, and addresses the manybiomedical and microfabrication applications for such a film.

The substantial difference in flexibilities demonstrated by the filmproduced pursuant to the invention and the film produced undercontinuous-PECVD conditions can be quantified based on a comparison ofthe average network connectivity of the films with the networkconnectivity corresponding to the so-called percolation of rigiditylimit. The average connectivity of a continuous random covalent networkcorresponds to a sum of the compositional fractions of the networkcorrespondingly weighted by the number of bonds of the variouscompositions. For the films produced in Example 1, the compositionincludes C-CF, CF, CF₂, and CF₃ ; the average connectivity of the filmsis a sum of the fractions of these compositions, weighted by 4, 3, 2,and 1, respectively, in correspondence to their bonding configuration.

A corresponding connectivity number for each of the films is thusdetermined based on the fractional areas of the deconvoluted peaks ofthe XPS spectrum for that film, whereby the fractions to which thebonding weights are to be assigned is determined. For the film producedunder continuous-PECVD conditions, an average connectivity number ofabout 2.6 is given, while for the film produced pursuant to theinvention, an average connectivity number of about 2.1 is given, basedon the spectra of FIGS. 3A and 3B, respectively. These connectivitynumbers are significant when compared to the so-called percolation ofrigidity limit. This limit characterizes a material's flexibility; abovethe limit, a material is characterized as being substantially inflexibleand over constrained, while below the limit, a material is characterizedas being flexible.

The average connectivity of a continuous random covalent network shouldbe below about 2.4 to be below the percolation of rigidity limit, i.e.,to be flexible. The film pursuant to the invention has an averageconnectivity of about 2.1, and thus is quantitatively characterized asbeing flexible. In contrast, the film produced under continuous-PECVDconditions has an average connectivity of about 2.6, and is thusquantitatively characterized as being an over constrained material. Thisquantitative analysis supports the wire bending test results, which alsoconfirm the improvement in flexibility achieved by films pursuant to theinvention.

To further examine the strength and flexibility of the film producedunder conditions pursuant to the invention, the loop coated with thisfilm was cut using a razor blade. FIG. 4C shows an ESEM view of the cutcross section. No sign or indication of shattering due to brittlenesswas found; the film retained its integrity as-cut.

Factors believed to influence the mechanical flexibility of PECVD filmsin general include film morphology, mismatch in the coefficient ofthermal expansion between the film and the underlying substrate, and thechemical composition of the film. In both the pulsed- andcontinuous-PECVD processes carried out for the experiment describedabove, the film growth rates were estimated to be about 10±2 Å/s, andthe wire temperatures during the deposition were assumed to be similardue to identical backside electrode cooling configurations and electrodegeometries. Accordingly, the morphology and degree of thermal mismatchfor both coated wire samples were assumed to be similar. This leads tothe conclusion that the chemical composition of the film produced by thepulsed-PECVD conditions is the determining factor for that film'sflexibility. Indeed, it is understood that the lower crosslink densityof the film produced by the pulsed-PECVD conditions, as well as thefluorine-rich composition of the film, provides the flexibilitydemonstrated by the film.

Nuclear magnetic resonance measurements were undertaken to examine theparticular morphology of the film produced by pulsed-PECVD conditions.It was found that the fluorocarbon thin film was substantially amorphousin nature. This is advantageous in that various film properties, such asdielectric properties, are accordingly isotropic in nature. In contrast,bulk PTFE was found to be partially crystalline in morphology. This bulkmorphology accounts for several unwanted characteristics, including verylow solubility. Films produced by the pulsed-PECVD process of theinvention thereby provide advantages in morphology over that of thecorresponding bulk material.

To investigate further materials properties of the film produced bypulsed-PECVD conditions, the refractive index of the film was determinedusing standard ellipsometry techniques. It was found that the refractiveindex of the film was about 1.36±0.03. The reported refractive index ofbulk PTFE is about 1.38. This measurement provides further evidence forthe chemical resemblance of the film produced by pulsed-PECVD conditionsto bulk PTFE.

EXAMPLE 2

A 0.5 μm-thick fluorocarbon film was deposited on a silicon wafersubstrate under the pulsed-PECVD conditions of Example 1; no adhesionpromoter was employed. The film was scored into 100 1 mm×1 mm squaresacross the substrate using a razor. The adhesion of the film to theunderlying substrate was tested by a conventional tape test. No failurefor any of the 100 squares occurred, indicating that the film provides avery high degree of adhesion; this quality is particularly important forcoating complex flexible structures that are bent during operation.

EXAMPLE 3

To further demonstrate the conformal coverage attainable by apulsed-PECVD process in accordance with the invention, a 9 μm-thickfluorocarbon film was deposited on an iridium neural probe under thepulsed-PECVD conditions of Example 1. The iridium probe was provided byHuntington Medical Research Institute (HMRI) of Pasadena, Calif.; suchprobes are employed, e.g., to measure electrical impulses emitted fromsensitive areas of the human brain. The probe included a cylindricalshaft of about 45 μm in diameter, tapering to an end tip of about 5 μmin diameter. During the deposition process, the probe was supported in abulk PTFE block in a configuration as described above. The probe'selectrical potential floated during the deposition, i.e., was allowed toequilibrate to the potential of the plasma. FIG. 5 shows an ESEM view ofthe coated probe. Note that the probe coating was uniform along theentire extent of the probe, including the very narrow tip portion. Thisindicates that the deposition process enables both conformability anduniformity along minute structures.

EXAMPLE 4

The strength, as well as conformability, of a fluorocarbon film producedby the pulsed-PECVD process of the invention was examined by depositinga 28 μm-thick fluorocarbon film on a silicon micro-ribbon cable. Thecable was about 100 μm-wide, about 1 centimeter-long, and about 12 μm inthickness, and was configured as a neural probe. The deposition processconditions of Example 1 were carried out, with the electrical potentialof the ribbon set at the floating potential of the plasma.

Prior to the deposition process, the micro-ribbon cable was manuallybent to form a 90 degree angle. This was accomplished by placing each ofthe two ends of the ribbon in a separate metal tube having dimensionssimilar to that of the ribbon. The two tubes were in turn positioned incorresponding holes drilled into a PTFE block. The tubes were used tobend the ribbon and maintain the desired 90 degree curvature. The bentstructure was then maintained in this position during the deposition bythe tubes and PTFE block configuration. At the end of the 28 μmdeposition process, the micro-ribbon cable was removed from the supporttubes. It was found that the cable maintained the 90 degreeconfiguration. This demonstrates that the deposition processes providedby the invention can be employed as casting techniques that not onlyaccomplish physical casting of a structure into a desired configuration,but that at the same time provide encapsulation of the structure with adesirable PTFE-like film that itself is the casting material. As will berecognized by those skilled in the art, the nature of the structure anddesired structural configuration, as well as other factors, sets thepreferable thickness of the casting film. Films as thin as a few micronsare acceptable for many applications.

A second deposition process is provided by the invention for forming afluorocarbon polymer thin film having materials properties like that ofbulk PTFE. In this second process, a thermal, rather than rf power,input excitation is employed to produce reactive gas phase species in achemical vapor deposition environment. This thermal-excitation processreduces the production of dangling bond defects in a deposited film toan even larger extent than the pulsed-PECVD process of the invention.Recall that dangling bond film defects result to a great extent fromfilm ion bombardment during a chemical vapor deposition process. Thepulsed-PECVD process of the invention significantly reduces such ionbombardment by employing intervals of plasma excitation, rather thancontinuous plasma excitation as is conventional. In the thermal chemicalvapor deposition (thermal-CVD) process provided by the invention,substantially no ion bombardment occurs, because no substantial electricfield is generated in the deposition chamber to attract the charged ionsto the film as it is deposited.

Referring now to FIG. 6, the thermal-CVD process can be carried out in avacuum deposition chamber substantially identical to that describedabove and shown in FIG. 1, with the addition of a heated surface, e.g.,a hot-filament 50, as shown in FIG. 6. The hot-filament or other heatedsurface is preferably provided in a position relative to the input feedgas flow such that the input feed gas flows in the vicinity of theheated structure; whereby the gas is pyrolyzed to produce reactivedeposition species. For example, as shown in FIG. 6, a hot-filament 50can be positioned just below a shower-head electrode 14, here unpowered,such that gas injected to the chamber through the shower-head electrodepasses over the hot-filament. The hot-filament can be heated by, e.g.,resistive heating. In this case, a dc voltage source 52 is provided toapply the heating voltage to the filament, consisting of, e.g., a Ni/Crwire.

The lower electrode 16, to which no electrical contact need be made inthis case, is preferably maintained at a temperature lower than that ofthe hot-filament such that reactive species produced in the vicinity ofthe filament are transported to the wafer, where they deposit andpolymerize. Cooling coils 31, or other appropriate cooling mechanism,can be employed to maintain a substrate 54 or other structure supportedon the lower electrode at a desired temperature.

Thermal excitation mechanisms other than a hot-filament are equallysuitable for the thermal-CVD process. Indeed, it is preferable that theselected thermal mechanism, together with the gas delivery system,provide both uniform gas input and uniform pyrolysis of the gas. Ahot-filament pyrolysis configuration described above may not, in allcases, provide the desired pyrolysis uniformity. Hot windows,electrodes, or other surfaces, as well as heated walls of the depositionchamber, can alternatively be employed in pyrolysis configurations aimedat producing uniform gas pyrolysis.

In one alternative, the upper shower-head 14 is itself heated, wherebyinput feed gas is pyrolyzed as it passes through the shower-head. Suchheating can be accomplished by, e.g., applying a dc voltage to theshower-head, which preferably consists of, e.g., aluminum or stainlesssteel. As the input feed gas is delivered from the feed gas source tothe heated upper shower-head, the gas preferably is maintained at atemperature at which it is not pyrolyzed, such that substantially allpyrolysis occurs only once the gas enters the shower-head. In this case,of course, an additional hot-filament is not needed in the depositionchamber.

In a similar alternative, the upper shower-head is outfitted with anarray of tubes, each shower-head hole having a tube protruding from it.Such tubes consist of, e.g., anodized aluminum or stainless steel Inthis case, the shower-head is not itself heated. Instead, the tubesprotruding from the shower-head are configured to fit into acorresponding array of holes in a heated plate suspended just below theshower-head, such that the tubes extend to some depth, e.g.,substantially through, the holes in the plate. In this configuration,gas in the shower-head passes from the shower-head through the tubes andthrough the heated plate, whereby the gas is heated at the lower surfaceof the plate as it exits the tubes. This produces a plane of pyrolyzinggas that is substantially parallel to the lower electrode. As a result,both uniform gas injection and uniform pyrolysis of gas is achieved.Accordingly, this enables production of a substantially uniform reactivegas species environment in the vicinity of an object to be coated by thedeposition process. As will be recognized by those skilled in the art,this pyrolysis configuration is therefore preferable for applications inwhich deposition uniformity is of importance, e.g., in the case ofdeposition on a large substrate or other object, or a spaced array ofsubstrates or other objects in, e.g., a production environment.Preferably, the tubes connected between the shower-head and the heatedplate are slightly smaller in diameter than the holes in the heatedplate, such that no substantial pyrolysis occurs until the gas exits thetubes and at the plate lower surface. Additionally, the heated plate ispreferable suspended slightly below the shower-head such that theshower-head is not substantially heated by the plate. The plate ispreferably formed of, e.g., aluminum or stainless steel, is thick enoughto produce uniform heating, and is heated by conventional techniques.

In another alternative technique, the input feed gas is heated in, e.g.,the gas delivery tube 56 connecting the gas feed source (18 in FIG. 1)to the upper shower-head 14. Here, the pyrolyzed gas is preferably pipedto the location of the substrate or other structure to be coated in amanner similar to that for conventional downstream-ashing processes. Inyet another pyrolysis configuration, a cold feed gas is mixed with a hotinert as, such as argon, in the deposition chamber. In this case, theinert gas is injected and heated by, e.g., one of the processesdescribed above. Mixture of the heated gas with the cold input feed gasresults in pyrolysis of the feed gas. This pyrolysis technique has theadvantage of eliminating a pyrolysis surface in the chamber that itselfis coated with the reactive gas species produced by the pyrolysis. Otherdirect heating techniques, e.g., laser heating techniques, can also beemployed, as can be employed, in general, a wide range of otherpyrolysis mechanisms.

Like the pulsed-PECVD process, the thermal-CVD process accommodatesfluorocarbon polymer thin film deposition on a wide range of substratesand three-dimensional structures. Accordingly, the wire holders andprobe support blocks described above, as well as other suitablestructural supports, can be employed for enabling film deposition on adesired structure. The only requirement imposed by the thermal-CVDprocess on structural supports is the ability to maintain a supportedstructure at a desired temperature that preferably is lower than thepyrolysis temperature.

The thermal-CVD process of the invention contemplates use of any feedgas that provides a monomer which can be pyrolyzed to providedifluorocarbene species (CF₂) for producing a fluorocarbon polymer filmhaving a high fraction of CF₂ groups and a low degree of polymercrosslinking. For example, the HFPO monomer described above isunderstood to decompose under pyrolysis to form a fluorinated ketone andthe desired difluorocarbene. The fluorinated ketone is relativelystable, compared with the difluorocarbene. This is understood to lead toa high CF₂ content in a film as polymerization occurs at the filmdeposition surface. Oxygen present in the monomer is tied up in therelatively unreactive ketone decomposition byproduct, whereby littleoxygen is incorporated into the film. Thus, as shown in this example, awide range of monomer gases, which may include various constituents, canbe employed to produce the desired fluorocarbon thin films. As will berecognized by those skilled in the art, the list of monomers givenabove, as well as other suitable monomers, can be employed.

As with the pulsed-PECVD process, the thermal-CVD process can beaccomplished under the wide range of process conditions givenpreviously. The lower electrode temperature is preferably maintained ata temperature lower than the pyrolysis temperature of the selectedmonomer gas. Specifically, the temperature of the lower electrode ispreferably maintained low enough to favor polymerization under thepartial pressure of a given reactive species employed in the depositionprocess. It is also preferable that the partial pressure of the reactivespecies be kept to a low level that prevents homogeneous gas-phasereactions, which could cause particle production in the gaseousenvironment rather than on the object surface to be coated. As with thepulsed-PECVD process, pre-adhesion promotion processes andpost-annealing processes can be paired with the thermal-CVD process.

Fluorocarbon polymer thin films produced pursuant to the thermal-CVDprocess of the invention are characterized by the flexibility andstructural integrity, as well as dielectric properties, of filmsproduced pursuant to the pulsed-PECVD process of the invention, and arefurther characterized by an even higher compositional CF₂ fraction andan even lower dangling bond concentration than that of films producedpursuant to the pulsed-PECVD process. Conformal coverage of a wide rangeof three-dimensional structures for providing a flexible encapsulatingstructural coating having PTFE-like properties is thus well-enabled bythe thermal-CVD process.

EXAMPLE 5

A fluorocarbon polymer thin film was produced in accordance with thethermal-CVD process of the invention by flowing 12.5 sccm of undilutedHFPO (from PCR, Inc.) of about 99% purity, into a parallel plate vacuumdeposition chamber, like that described above, maintained at a pressureof about 1 Torr. A hot-filament of Ni/Cr wire was positioned under theupper shower-head. Using a dc voltage, the hot-filament was maintainedat a temperature of about 673K. A water cooling arrangement was employedto maintain the lower chamber electrode at a temperature of about293±3K. A silicon substrate was supplied on the cooled electrode todeposit a fluorocarbon polymer film on the substrate.

FIG. 7 shows a plot of the carbon-1s X-ray photoelectron spectrum forthe deposited film. The dominate peak at 292 eV is indicative of the CF₂bonding environment; indeed, deconvolution indicates that CF₂ bondingaccounts for about 90% of the spectra. Comparing this plot with that ofa film produced by the pulsed-PECVD process of the invention, as shownin FIG. 3B, it is seen that the film produced under thermal-CVDconditions has a higher CF₂ content; the film produced underpulsed-PECVD conditions is characterized by a CF₂ fraction of about 65%.It must be noted, however, that both the pulsed-PECVD and thermal-CVDprocesses produce films having a marked increase in CF₂ fraction overfilms produced by, e.g., continuous-PECVD conditions. As shown in FIG.3A, a film produced under continuous-PECVD conditions is characterizedby a CF₂ fraction of only about 21%. Recall that bulk PTFE, the spectrumfor which is shown in FIG. 3C, is characterized by substantially 100%CF₂, neglecting end groups, which account for only about 1/10,000 of thecompositional units. The small peak in the bulk PTFE spectrum that isnot CF₂ is likely to be due to carbon contamination from absorbedatmospheric hydrocarbons rather than from the bulk PTFE compositionitself.

The crosslinking density of the film produced under thermal-CVDconditions was determined based on deconvolution of the XPS plot, in themanner given above in Example 1. It was found that crosslinking lowerthan about 15%, and even as low as about 5%, is obtainable by thethermal-CVD process. This very low crosslinking density enables a highdegree of flexibility in the deposited films.

Based on these results, it is clear that the thermal-CVD process of theinvention provides a fluorocarbon polymer thin film having a chemicalcomposition that very closely resembles that of bulk PTFE. As with thepulsed-PECVD process of the invention, this enables a wide range of thinfilm applications for PTFE that heretofore have been only sub-optimallyaddressed by conventional polymer films.

To investigate further materials properties of the film produced bythermal-CVD conditions, the refractive index of the film was determinedusing standard ellipsometry techniques. It was found that the refractiveindex of the film was about 1.35±0.03. The reported refractive index ofbulk PTFE is about 1.38. This measurement thus provides further evidencefor the chemical resemblance of the film produced by thermal-CVDconditions to bulk PTFE.

EXAMPLE 6

A silicon substrate was subjected to the thermal-CVD process conditionsof Example 5, with the hot-filament temperature held at about 691K. Itwas found that the film deposition rate under these conditions was about1.8 μm/hour. With a deposition cycle of several hours, a film thickerthan about 10 μm was deposited. This demonstrates that a wide range ofcoating thicknesses can be provided by the thermal-CVD process inreasonable and practical processing times.

EXAMPLE 7

A length of stainless steel wire having a diameter of about 25 μm wassubjected to the thermal-CVD process conditions of Example 5. A wireholding ring like that shown in FIG. 2 was employed to support the wireduring the deposition process. The deposition was carried out to producea wire coating thickness of about 16 μm. The structural integrity of thecoating was tested by cutting through the thickness of the wire with arazor blade. FIG. 8 shows an ESEM view of the cut cross section. Asshown in the figure, the film coating maintained its integrity as-cut;no sign or indication of shattering due to brittleness was found. Thecoating produced by thermal-CVD conditions is thus demonstrated toexhibit the superior structural characteristics of bulk PTFE.

EXAMPLE 8

Fluorocarbon films of about 0.6 μm in thickness and about 8 μm inthickness were deposited on silicon substrates under the thermal-CVDconditions of Example 5; no adhesion promoter was employed. The filmswere scored into 100 1 mm×1 mm squares across the substrates using arazor. The adhesion of the films to the underlying substrates was testedby a conventional tape test. No failure for any of the squares occurred,indicating that over a wide range of film thicknesses, a very highdegree of adhesion is provided by the thermal-CVD process; this qualityis particularly important for coating complex flexible structures thatare bent during operation.

EXAMPLE 9

Electron spin resonance (ESR) measurements were undertaken to ascertainthe dangling bond density of a film produced under thermal-CVDconditions, and to compare this density with that of films producedunder pulsed-PECVD conditions and continuous-CVD conditions. A film wasproduced using the thermal-CVD process of Example 5 with thehot-filament temperature maintained at about 648K. A film was alsoproduced under the pulsed-PECVD conditions of Example 1.

In the ESR measurements, it was assumed that the deposited films havethe same density as that of bulk PTFE, which is about 2.2 g/cm³. For thefilm deposited under thermal-CVD conditions, a dangling bond density ofabout 1.2×10¹⁸ spins/cm³ was determined. These defects have a g-value ofabout 2.0108 and a large linewidth of about 60 G. For the film depositedunder pulsed-PECVD conditions, a dangling bond density of between about0.8×10¹⁸ -13×10¹⁸ spins/cm³ was determined. In contrast, the danglingbond density of films produced under continuous-PECVD conditions isreported to typically range between 10¹⁸ -10²⁰ /cm³. Thus, filmspursuant to both the thermal-CVD process and pulsed-PECVD process of theinvention have a relatively low dangling bond density compared to thatof conventional PECVD polymer films.

It must be noted that the dangling bond density of a given film producedunder thermal-CVD conditions is a function of the pyrolysis mechanism.For example, the pyrolysis mechanism of Example 5, namely ahot-filament, is understood to introduce metal atoms into the depositingfilm as a result of evaporation of metal atoms from the filament duringthe pyrolysis. Accordingly, some amount of the ESR spin density givenabove for the film produced under these conditions is understood to beattributable to hot-filament metal evaporation.

As discussed earlier, various direct-pyrolysis mechanisms, and otherheating mechanisms, can be employed to pyrolyze the input feed gaswithout the use of a hot-filament. These mechanisms reduce or eliminatemetal contamination of a depositing film, and correspondingly arecharacterized by a lower dangling bond density, e.g., a density lessthan about 10¹⁷. Accordingly, one of these pyrolysis mechanisms ispreferably employed in applications for which a low dangling bonddensity is desired.

Turning now to other fluorocarbon polymer thin film deposition processesin accordance with the invention, there is contemplated a wide range ofhybrid PECVD/thermal-CVD deposition processes for customizing materialsproperties of a film in situ during the film deposition process. In afirst such process in accordance with the invention, two or moredeposition intervals are defined, each interval employing one or bothPECVD and thermal-CVD conditions. For example, during a depositioninitiation interval, one of continuous- or pulsed-CVD conditions areprovided; then during a growth phase interval, thermal-CVD conditionsalone or in combination with PECVD conditions are provided; followed bya final interval during which continuous- or pulsed-CVD conditions areprovided.

Processes such as this example three-interval process provide severaladvantages. First, it is recognized that ion bombardment aids ininitiation of deposition of gas species onto a substrate or otherstructure, due, e.g., to the electric field bias inherent in the plasmaexcitation conditions. This in turn enhances the adhesion of thedepositing film to the underlying substrate or structural surface. Thus,the ion, neutral, and free radical production provided by a plasmaprocess can be advantageously employed at the start of the deposition toaid in film nucleation and to enhance film adhesion. The film surfaceroughness characteristic of PECVD deposition conditions also aids inadhesion. Although the pulsed-CVD process conditions produce superiorresults, continuous-CVD conditions can also be employed for thisinitiation interval.

During a sequential growth phase, the plasma is extinguished and the gasheating is commenced, whereby only thermal processes produce thereactive gas species that polymerize on the surface of the substrate orstructure. As explained in detail above, such a thermal process producesa film having a very low dangling bond concentration, a relatively highCF₂ fractionality, and a low degree of crosslinking. These propertiesresult in a film having material properties that very closely resemblethose of bulk PTFE.

During the thermal-CVD growth interval, the properties of the film canbe further customized. For example, a plasma can be ignited for one ormore brief sub-intervals to reduce film crystallinity, to enhancecrosslinking, or to otherwise modify the film characteristics in adepth-dependent manner. Alternatively, a relatively low-power plasma canbe maintained during either the entire duration or a portion of thegrowth interval duration; results similar to that produced by thesub-interval plasma sequence are here achieved as a result of thecorrespondingly relative low level of ions. As in the initiationinterval, either continuous- or pulsed-PECVD can be employed in eitheralternative.

During the final deposition process interval, plasma conditions areprovided until the end of the deposition cycle. This results in adeposited film surface topology that generally enhances the adhesion ofa second film material to be subsequently deposited on the fluorocarbonfilm. In this final deposition process interval, the feed gascomposition can also be selected to enhance film surface adhesioncharacteristics. For example, oxygen and/or silicon-bearing gases can beadded to the feed gas composition to produce a film surface havingtopological and chemical properties that are conducive to adhesion aswell as providing, e.g., chemical stability such as oxidationresistance.

The three-interval deposition process just described is but one exampleof a wide range of process variations contemplated by the invention. Aswill be recognized by those skilled in the art, other combinations ofthermal-CVD and PECVD conditions can be employed to achievecustomization of a fluorocarbon polymer thin film as the film isdeposited. The degree of crosslinking, density of dangling bonds, andfractional composition of CF₂ desired for a given film can be controlledby way of the thermal-CVD and PECVD process combinations. Depositionrate can also be controlled by process PECVD and thermal-CVDcombinations; for example, deposition rate can be increased by providingPECVD and thermal-CVD conditions simultaneously. The examples discussedearlier provide a guide to selection of corresponding processparameters.

Whatever sequential or simultaneous conditions are employed, theypreferably are selected based on a given application. For example, athree-interval process of PECVD, thermal-CVD, and then PECVD conditionscan be employed to produce a graded interface between an underlyingsubstrate or layer, a fluorocarbon film having characteristics ofthermal-CVD processing, and an overlying layer. This enablescustomization of the lower and upper interfaces to accommodate varyingmechanical and chemical conditions. As will be recognized by thoseskilled in the art, many other film configurations can be addressed bythe combination processes provided by the invention.

EXAMPLE 10

A fluorocarbon polymer thin film was produced in accordance with thecombination thermal/PECVD process provided by the invention on a siliconsubstrate. In the process, a hot-filament at a temperature of about 691Kwas employed to pyrolyze HFPO monomer gas flowing at about 12.5 sccminto the deposition chamber. The substrate was supported on the lowergrounded electrode, which was back-side cooled to a temperature of about293±3K. A chamber pressure of about 1 Torr was employed; the chamberconfiguration was in other respects the same as described above.Simultaneously with the pyrolysis mechanism, a pulsed-PECVD environmentwas provided. This was accomplished with an rf power density of about2.7 W/cm² ; the pulsed plasma excitation duty cycle consisted ofexcitation on-time interval of about 10 milliseconds and an off-timeinterval of about 400 milliseconds.

Compositional XPS characterization of the produced film indicated thatat least about 85% of the film was composed of CF₂ groups, with CF₃ andCF groups making up only about 15% of the composition. The index ofrefraction of the film was found to be about 1.35±0.03. Thus, the filmwas found to exhibit properties very close to that of bulk PTFE.Furthermore, the combination process was found to be characterized by adeposition rate of at least about 1.8 μm/hour. This indicates that thecombination process is viable for production environments.

Considering applications of the various fluorocarbon polymer thin filmsprovided by the invention, the examples discussed above clearlydemonstrate the advantageous use of the films as flexible PTFE-likecoatings for biomedical devices like implantable probes and devicewires. Heretofore, the high degree of crosslinking and low CF₂compositional fraction characteristic of conventional polymer films haverendered such films inferior as coatings for biomedical devices. Thedemonstrated flexibility and mechanical integrity, along with theconformability and chemical stability of films pursuant to the inventionnow enable encapsulation of biomedical devices with a PTFE-like coatingthat can be relied upon even in such crucial applications.

Of course, the encapsulation process provided by the invention can beextended beyond biomedical applications to any application for which aPTFE-like coating is desired. For example, razor blades such as thoseused conventionally for shaving, can be conformally coated with filmspursuant to the invention to lubricate the razor blade surface.

Films pursuant to the invention can also be used for producing alow-friction coating on mechanical parts of both macroscopic andmicroscopic dimensions. The deposition processes provided by theinvention are especially well-suited for providing low-friction coatingson microfabricated moving parts, e.g., micro machined sensor andactuator structures. Such structures are typically manufactured usingmicrofabrication processes, such as vapor-deposition processes, andtypically employ microfabrication materials, such as silicon, that arecompatible with such processes. The deposition processes provided by theinvention enable coating and encapsulation of moving micromechanicalstructures to reduce friction typically exhibited by such structures.

Films pursuant to the invention can also be employed as separationmembranes to, e.g., filter various chemical species out of a gas orliquid that is delivered through the membrane. The low waterpermeability of the films enables separation of gases out of a stream ofwater- or steam-containing media. The low water permeabilitycharacteristics of the films can also be employed simple as a coating toreduce the water permeability of a structure, e.g., a porous structure.As will be recognized by those skilled in the art, there exists a widerange of alternative applications that are well-suited for thecharacteristics of films pursuant to the invention for addressing thinfilm applications of bulk PTFE.

The fluorocarbon polymer thin films of the invention find particularlysuperior application to semiconductor microfabrication processing.Microfabricated integrated circuit designs increasingly call forreduction of circuit signal propagation delay, reduction of circuit anddevice power consumption, and reduction of cross-coupling of circuitsignals and noise signals in adjacent circuit connections, among otherincreasing requirements. All of these requirements point to use ofinterlayer dielectric materials having reduced dielectric constants.

Although various organic polymers and microporous materials have beeninvestigated as low dielectric constant (low-k) materials, these haveall fallen short of the range of necessary film properties. For example,in addition to low-k, interlayer dielectric materials are increasinglyrequired to possess high dielectric strength, high surface and bulkresistivity, low stress, high thermal stability, low permeability towater, and good adhesion. In addition, the materials preferably areuniform and pin hole-free, and can preferably be photolithographicallypatterned. Furthermore, and importantly, due to increasingly complexcircuit topology, interlayer dielectric materials are required to beconformal, i.e., to provide good step coverage over, e.g., metalizationlayers. Spin-applied materials, e.g., polyimides, that have beenconsidered as interlayer dielectric materials are as a practical matter,not typically acceptable because they cannot meet this conformabilityrequirement. Indeed, as circuit line widths decrease to less than 0.25μm, spin-applied materials are not practical. Furthermore, multiplespin-application steps can be required to adequately cover substantialsurface topology.

The fluorocarbon polymer thin films provided by the invention areideally suited as vapor-deposited interlayer dielectric materials formicrofabrication applications. Each of the materials requirements justoutlined are met by the films, and improvements in materials propertiesare further provided. CVD processes are inherently compatible withconventional microfabrication processing, and can be configured in asequence of processing steps that are all carried out in a singledeposition chamber or a cluster of chambers. Indeed, the nature of thevacuum deposition chamber accommodates in situ pre-deposition adhesionpromoter deposition steps, post-deposition surface-modification steps,and incorporation of feed gas constituents at intervals in thedeposition.

Many advantages over conventional interlayer dielectric materials arefurther provided by the thin films of the invention. For example, unlikespin-applied films, the vapor-deposited films of the invention arecharacterized by isotropic dielectric properties, due to their generallyamorphous character. In addition, the vapor-deposited films do notcontain residual solvent, as is common for spin-applied materials.

It has been generally accepted that films deposited under PECVDconditions are unacceptable as an interlayer dielectric material becausesuch films are typically characterized by a significant concentration ofdangling bonds. As explained above, both the pulsed-PECVD andthermal-CVD processes of the invention provide films having dramaticallyreduced dangling bond concentrations. As a result, films pursuant to theinvention have reduced dielectric loss, have greatly diminishedreactivity with atmospheric water and oxygen, and generally are farsuperior both chemically and structurally to prior PECVD films. As willbe recognized by those skilled in the art, the films pursuant to theinvention thus are also excellent as microfabricated circuit barrier andencapsulation layers.

In an example microfabrication sequence, a first layer of metal isdeposited on a microfabricated circuit configuration at an appropriatepoint in the fabrication sequence. The metal layer can be patterned andetched using photolithographic and etch techniques to form desiredconducting circuit interconnections. Then, if desired, an adhesionpromoter can be applied to the patterned metal, using, e.g., one of theadhesion promoters discussed above. Next, a deposition process inaccordance with the invention, e.g., a three-interval process consistingof a first pulsed-PECVD interval, a thermal-CVD interval, and a finalpulsed-PECVD interval, is carried out. The resulting fluorocarbonpolymer thin film can then be patterned using, e.g., a direct-writeprocess or photolithographic and etching process such as, e.g., anoxygen plasma to form desired interlayer pattern. A chemo-mechanicalpolishing process, or other dielectric layer planarization process, canthen be carried out provide a planar film surface, if desired.

At this point, a second layer of metal can be deposited, and the cycleof metal layer-interlayer dielectric deposition and patterning continueduntil a desired number of metal layers is achieved. As will berecognized by those skilled in the art, many variations can beincorporated into this process, including, e.g., planarization aftereach metal deposition step, as well as other fabrication processes.

From the foregoing, it is apparent that the fluorocarbon polymer thinfilm deposition processes provided by the invention enable production ofthin films that are characterized by a low degree of crosslinking, a lowdensity of dangling bonds, and a high compositional CF₂ fraction,leading to a very low dielectric constant, conformability, flexibility,and high structural and chemical stability. The invention therebyprovides the ability to produce a thin film having materials propertiesthat closely resemble those of bulk PTFE. It is recognized, of course,that those skilled in the art may make various modifications andadditions to the example deposition processes described above withoutdeparting from the spirit and scope of the present contribution to theart. Accordingly, it is to be understood that the protection sought tobe afforded hereby should be deemed to extend to the subject matterclaims and all equivalents thereof fairly within the scope of theinvention.

We claim:
 1. A method for forming a fluorocarbon polymer thin film on asurface of a structure, comprising the steps of:exposing a monomer gasto a source of heat having a temperature sufficient to pyrolyze themonomer gas, the monomer gas selected to produce upon pyrolysis a sourceof reactive species that includes polymerizable CF₂ species and thatselectively promotes CF₂ polymerization, the reactive species sourcebeing in the vicinity of a structure surface on which a fluorocarbonpolymer thin film is to be formed; and maintaining the structure surfacesubstantially at a temperature lower than that of the heat source toinduce deposition and polymerization of the CF₂ species on the structuresurface.
 2. The method of claim 1 wherein the monomer gas compriseshexafluoropropylene oxide.
 3. The method of claim 1 wherein the heatsource to which the monomer gas is exposed comprises aresistively-heated conducting filament suspended above the structuresurface.
 4. The method of claim 1 wherein the heat source to which themonomer gas is exposed comprises a heated plate having a pyrolysissurface that faces the structure.
 5. The method of either of claims 3 or4 wherein the heat source temperature is greater than about 500K, andwherein the step of maintaining the structure surface temperaturecomprises maintaining the structure surface at a temperature less thanabout 300K.
 6. The method of claim 1 wherein the structure comprises alength of wire.
 7. The method of claim 1 wherein the structuremicrofabrication comprises a substrate.
 8. The method of claim 1 whereinthe structure comprises a neural probe.
 9. The method of claim 1 whereinthe structure comprises a razor blade.
 10. The method of claim 1 whereinthe structure comprises microstructure having multiple surfaces all ofwhich are maintained substantially at a temperature lower than that ofthe heat source.
 11. The method of claim 1 further comprising a firststep of applying plasma excitation power to the monomer gas.
 12. Themethod of claim 11 further comprising a last step of applying plasmaexcitation power to the monomer gas.
 13. The method of either of claims12 wherein the monomer gas is not substantially pyrolyzed during plasmaexcitation power application.
 14. The method of claim 1 wherein the stepof exposing the monomer gas to a heat source further comprisessimultaneous application of plasma excitation power to the monomer gas.15. The methods of any of claims 11, 12, or 14 wherein the appliedplasma excitation power is characterized by an excitation duty cyclehaving alternating intervals in which excitation power is applied and inwhich no excitation power is applied to the monomer gas.